Acquired equivalence reflects conditioning processes underlying the ability to categorize physically dissimilar events into groups and, with it, the ability to respond appropriately to new events or new combinations of events without direct training. The goal of this proposal is to identify some of the origins of acquired equivalence classes, their effects on stimulus control, and the nature of the events that may join them. Specifically, the proposed experiments will study the role of common responses in generating classes of equivalent stimuli and serving as indices for them, and examine the potential for the responses themselves to become class members. Pigeons will be trained on a variety of two-choice conditional and simple discriminations in which multiple stimuli (including those arising from their own behavior) occasion the same subsequent response or response pattern. Afterwards, class formation will be assessed through transfer-of-control tests that have been effective in demonstrating acquired equivalence and other forms of stimulus control in previous studies of animal cognition and human categorization. Besides providing a clearer picture of how common responses promote and reveal acquired equivalence classes, the proposed work will provide data relevant to a recent hypothesis that equivalence classes in general may arise directly from reinforcement contingencies and, thus, include responses as members. The results will also provide important and needed clarifications of recently reported equivalence-like effects of potential theoretical import and will create additional points of contact and comparison between animal categorization and the behavior-analytic literature on emergent behavior. [unreadable] [unreadable]